1. Field of the Invention
The invention relates generally to spread spectrum signal receivers and more particularly to a method and apparatus for improving signal-to-jamming ratio for a spread spectrum signal in the presence of a CW jamming signal.
2. Description of the Prior Art
Spread spectrum signals are used in global positioning system (GPS), global orbiting navigational system (GLONASS), and code division multiple access (CDMA) wireless communication systems. It is often the case that these signals must be received in the presence of a CW jamming signal. Depending upon the ratio of the power in the desired signal to the power in the jamming signal and the jamming tolerance of the signal receiver, the CW jamming signal may degrade the performance of the receiver or even make it inoperable. The term "CW" is used for describing any jamming signal whose modulation spectrum is much less than the modulation spectrum of the spread spectrum signal even though the jamming signal is not truly a "constant wavelength". An out-of-band CW jamming signal may be effectively eliminated by traditional filtering methods. However, an in-band CW jamming signal is more difficult to eliminate. One of the benefits of spread spectrum signal systems over traditional non-spread spectrum systems is that an in-band CW jamming signal is spread in frequency by a de-spreading process in a spread spectrum receiver, thereby improving the performance of the spread spectrum receiver in the presence of the jamming signal. Further improvement of the spread spectrum receiver is available by using multi-bit digitizing of the incoming signal combination. However, multi-bit digitizing is more complex and a very large in-band CW jamming signal may still degrade the performance of the spread spectrum receiver by overwhelming the de-spreading process. Theoretically, a notch filter having capacitive and/or inductive filter elements may be used to reduce the in-band CW jamming signal. However, the notch filter may need to be extraordinarily narrow and deep in order to be effective and may need to be tunable in operation to the particular frequency of the CW jamming signal that is present. Such notch filter is difficult and expensive to realize in practice.